I bring this site up a lot, and wanted to just share it again because it has a wealth of material that may help those of you doing 2D games: Chris Hildebrand's blog http://www.2dgameartguru.com/

A lot of the material is focused on character art and animation but the techniques will apply to environment art as well. I recommend starting at the beginning, in his 2011 Post Archive, which covers a lot of fundamentals and basic techniques.

Pay particular attention to the final steps in the tutorials, how he goes from the basic shapes to a finished look by introducing shading, highlights, textures, and other detail.